2. THE ISLAND UNIVERSE THEORY

The idea that our Sun is just one of myriads of stars in a huge stellar
system, the Milky Way, and that there may be many other stellar
systems of equal rank outside the Milky Way can be traced back to
the early eighteenth century
(1).
These early speculations (1)
by the Swedish philosopher, Emanuel Swedenborg
(2)
had no basis in actual
observations. Yet they are remarkably close to the present-day views
of the cosmos. Thomas Wright of Durham in England, writing at the
middle of the same century
(3),
conceived the idea apparently
independently and was the first to appeal to observational evidence in
support of it (2).
The best-known of the early exponents of the 'island
universe' theory are the German philosopher Immanuel Kant
(4), who
acknowledged his indebtedness to the ideas of Wright, and the English
astronomer, Sir William Herschel, who was the first to bring observational
techniques to bear specifically on the study of nebulae and
clusters of stars. In contrast, Messier compiled his catalogue of
nebulae and clusters, which predates Herschel's work, primarily as a
list of 'nuisances' to be avoided in his search for new comets
(5).

At first it was not possible observationally to distinguish between
gaseous nebulae and stellar aggregations, and it was thought that all
nebulae were groups of stars - some of them so distant that individual
stars could not be resolved. Since Herschel
(6)
was prepared to allow
a wide range of nebular sizes, from those comprising only a few stars
to systems of equal rank to the Milky Way (among which he included
M33, as well as M17, M31, and Orion), he attempted to estimate the
distances to nebulae on the basis of the amount of incipient resolution
into stars. Then he found
(7)
the planetary nebula NGC 1514 which, from its appearance
(8),
was obviously a single star associated with
faint nebulosity, a 'nebulous star'. For this reason, and from the fact
that star counts in selected regions ('gauges') did not enable him to
penetrate to the outside of the Milky Way, Herschel totally revised
his thinking, now maintaining
(9)
that all nebulae were small compared
to the Milky Way and were encompassed within it. The controversy
between the two views was not to be resolved for more than a century
after his death in 1822.

The observations by the Earl of Rosse with his 72-inch reflector
raised the fortunes of the island-universe theory again. In 1845 he
found the spiral nature of M51. His paper in 1850
(10) contained
drawings of five spiral nebulae, including M33 (see Plate I), and listed
fourteen more (3). He
also remarked that some of these spirals could be resolved into stars
(4). On this
basis, and using the star counts made by the Herschels, Alexander
(12)
suggested 'that the Milky Way and
the stars within it together constitute a spiral with several (it may be
four) branches', and traced the branches across the sky. But, in his
essay on the nebular hypothesis
(13),
Herbert Spencer argued that
since all stars are of similar sizes, all nebulae should be of similar
sizes; and if the nebulae are actually stellar aggregations, then the
apparently largest and presumably closest should be resolvable while
the smallest and farthest should remain unresolved. In arguing that
many quite small nebulae could in fact be resolved, Spencer mistook
small clusters for nebulae. However, this was not immediately
recognized.

Plate 1. Plate XXXVI. Fig. 5, H. 131 - This
figure represents the central portion
of a very large nebula. The nebula itself has not been sufficiently
examined,
but as yet no other portion appears to have a spiral, or indeed any regular
arrangement. The sketch is not very accurate, but represents
sufficiently well the general character of the central portion.

'September 6, 1849 - A spiral.

'September 16, 1849 - New spiral;
the brightest branch;
faint;
short
but pretty bright;
pretty
distinct; but
suspected; the whole involved in
faint nebula which probably extends past several knots which lie about
it in different directions. Faint nebula seems to extend very far
following: drawing taken.

Plate IIa. Drawing and photograph of
Messier 33. The drawing shows how
M33 appeared to the Earl of Rosse through his
72-inch reflecting telescope
(11).

Plate IIb. The photograph, reproduced to the
same scale as the drawing, was
obtained with the Mt Palomar 48-inch Schmidt telescope on an emulsion
sensitive to red light.

In 1864, Sir William Huggins
(14)
observed several nebulae with a
spectroscope, finding first an emission-line spectrum for the planetary
nebula NGC 6543 and, a few nights later, a continuous
spectrum with
a hint of absorption features for the great Andromeda nebula. This
showed that nebulae could be divided into two fundamentally distinct
classes: those that were clouds of luminous gas and those that were
stellar aggregations. All of the spiral nebulae were found to belong to
the latter class. The conclusive evidence of this was presented in 1899
by Schemer (15)
in the form of the first good-quality spectrogram of a
spiral nebula, M31.

The application of photography to studies of nebulae at the turn of
the century made it possible to get more detailed pictures of the form
of the spiral nebulae. The pioneering work in this field was done by
the Irish amateur astronomer Isaac Roberts
(16a,
b)
and by J.E. Keeler
with the 36-inch Crossley reflector of the Lick Observatory
(17). On
the basis of these photographs, the spiral nebulae were subclassified
into a 'condensed' type, showing condensations or knots of incipient
resolution over the entire face of the nebula, and an 'uncondensed'
type, showing incipient resolution only in the outermost parts of the
nebula, if at all. The prototypes of the two groups were M33 and M31, respectively. Dreyer
(18),
using photographs taken by Roberts,
published a catalogue of the condensations in M33 with the intent that it
form a basis for determining whether there were any changes in the
nebula in the course of time. Von der Pahlen
(19)
attempted to fit the
forms of the arms of spirals to mathematical curves. He found that
the two main arms of M33 fit logarithmic spirals quite well, but that
they had different rates of coiling and that they were not 180°
apart. Wolf and Ernst
(20a,
b)
catalogued 517 knots of nebulosity in a region centred on M33, extending over a diameter of several degrees but
omitting the portion within 25' of its nucleus. They found that the
nebulae tended to concentrate in patterns which looked like extensions
of the spiral arms so prominent in the inner part of M33. They
claimed that the spiral pattern could be followed over a region
8° in diameter and concluded that M33 must therefore be quite nearby.

In 1914, Slipher (21)
reported a radial velocity of -300 km s-1 for
M31, and Wolf
(22)
reported the first spectrographic measurements of
internal motions of spirals, finding rotational velocities of 100 km
s-1 in M81. In a larger-scale survey of the motions of spirals,
Slipher
(23)
noted that their average velocity was roughly 25 times as large as the
average stellar velocity, larger than for any other class of astronomical
objects. He also found large internal motions, evidenced by a rotational
velocity of the edge-on spiral NGC 4594 of 100 km s-1
20" from the nucleus. Pease
(24),
(25)
confirmed the finding of large internal
motions in spirals by measuring a difference of roughly 200 km
s-1
between the radial velocities of the nucleus of M33 and of NGC 604, a
bright condensation 10' from the nucleus. The large systemic radial
velocities indicated that the spirals could not be gravitationally bound
by the Milky Way, and were thus at least not permanent members of it.
The radial-velocity measurements of internal motions, if they could
be combined with proper-motion data, would provide a direct
measurement of the distances of the spirals.

During the first few years the Mt Wilson 6o-inch telescope was in
operation, Ritchey had obtained the best nebular photographs yet.
His pictures of the condensed type of spirals showed many points in
the nebulae which could be measured for proper motion. Second
epoch plates were taken and measured by van Maanen. In 1916 he
reported (26)
finding motions in M101 which could be interpreted
either as rotation or as outward streaming along the spiral arms. In
either case, the fact that proper motions could be measured in the
spirals indicated that they were nearby objects. His paper on M33
(27)
contains a summary of his measurements and interpretations. To
provide a check on these critical measurements, Lundmark
independently remeasured the more than 400 points on van Maanen's
plates of M33. Although his results correlated well with van
Maanen's
with respect to both direction and relative size of proper motion, the
absolute scale of Lundmark's proper motions was less than 1/10 as
large as those van Maanen had measured
(28).

Meanwhile, evidence supporting the extragalactic interpretation of
spiral nebulae began to mount up. In 1917, Ritchey's first photographic
discovery of a nova in a spiral nebula
(29),
spurred Ritchey
(30) and
Curtis (31)
to examine the large plate collections of Mt Wilson
and Lick Observatories, where they found a score of others that had
gone undetected in previous years, including two in M31. Systematic
surveys over the next two years produced 14 additional novae in M31,
but none in any other spiral. These formed a homogeneous group, all
peaking at about mpg = 17, about 10 magnitudes fainter
than the visually detected nova S Andromedae of 1885 in M31. S And, on the
other hand, was the only nova in M31 which conformed to the pattern
of the novae discovered in other spirals, all of which had light outputs
which were appreciable fractions of the total light outputs of the
associated nebulae. At this time there was no compelling reason to
choose either type of nova to represent the analogue of the galactic novae.

In 1921, Lundmark (32)
found that counts of stars brighter than
mpg = 15.7 in the field of M33 were not influenced by its presence.
Hence M33 had to be farther away than the average
15.7m star, roughly
3 kpc. Moreover, if the brightest stars in M33 had absolute magnitudes
of -6, it had to be 300 kpc away. In 1922, Duncan
(33) discovered
three recurrent variable stars in M33, the first found in any spiral.
He also reported a suspected nova. By the end of 1924, Hubble
(34)
had assembled data on 47 variables in M33, 22 of which were Cepheids
obeying the same period-colour-index and period-amplitude relations
as the Cepheids in the Milky Way and the Magellanic Clouds. Applying
the period-luminosity law, Hubble derived a distance of 285 kpc to
M33, and a comparable result from similar data for M31. These
distances agreed with those derived from the brightest non-variable
stars and with those derived by choosing to identify the fainter of the
two types of novae in M31 with galactic novae; S And and the novae
in the other spirals were called supernovae. Star counts in M33
(35),
coupled with the Cepheid distance scale, showed that its luminosity
function agreed with Kapteyn's luminosity function for the Milky Way,
and thus appeared to settle the island-universe controversy. The one
remaining loose end was the question of the reality of van Maanen's
rotations. This was decided negatively on the basis of plates of M33
and M74 from the Mt Wilson 100-inch telescope
(36a,
b,
c).

1 Swedenborg wrote in the first chapter
of the third part of his Principia:

'The common axis of the sphere or starry heaven seems to be the galaxy,
where we perceive the greatest number of stars.... There may be
innumerable spheres of this kind or starry heavens in the finite
universe. These may be associated one with the other... and the whole
visible starry heaven is perhaps but a point in respect to the
universe. The objects comprehended within the range of our bodily vision
are perhaps few; the greater number can be comprehended only by the
mind. This very starry heaven, stupendous as it is, forms, perhaps, but
a single sphere, of which our solar vortex constitutes only a part; for
the universe is finited in the infinite. Possibly there may be other
spheres without number similar to those we behold; so many indeed and so
mighty, perhaps, that our own may be respectively only a point; for all
the heavens, however many, however vast, yet being but finite, and
consequently having their bounds, do not amount even to a point in
comparison with the infinite.'

An interesting sidelight is cast by this excerpt from the following
chapter, which foreshadows the idea of continuous creation contained in
the hypothesis of the steady-state universe:

'Nor can she [nature] relatively equal or occupy a point of the
infinite. Hence new heavens one after the other may arise; in these
heavens, new vortices and world-systems; in these world-systems, new
planets; around the planets, new satellites; and in this manner, at the
will of the Deity, new creations may arise in endless succession.'
Back.

2 'That this in all Probability may be
the real Case, is in some Degree made evident
by the many cloudy Spots, just perceivable by us, as far without our
starry Regions, in which tho' visibly luminous Spaces, no one Star or
particular constituent Body
can possibly be distinguished; those in all likelihood may be external
Creation, bordering upon the known one, too remote for even our
Telescopes to reach.'
Back.

3 Rosse's second paper
(11)
contains a drawing of M33 in which the spiral pattern
is delineated over a region ~ 20' in diameter, in good
agreement with the most
prominent arms seen on modern photographs. See
Plate IIa and IIb.
Back.

4 While it is conceivable that Rosse
could have resolved the brightest individual stars in M33, in the more distant spirals (and probably also
in M33)
what he thought were stars must have been compact clusters or associations.
Back.